As is widely known, solar cells that can convert the energy of light into electricity have been attracting much attention as a future clean energy source. In particular, dye-sensitized solar cells using a sensitizing dye are easy to produce, and also such cells can be provided with colorability, flexibility, etc. In this respect, they have different features from solar cells using silicon or an inorganic compound, and various intensive researches and developments have been made. As a result, some of solar cells using a sensitizing dye made of a Ru complex (Ru-based dye), for example, have achieved energy conversion efficiency (II) of more than 10%, and are expected to be put into practical use (Nonpatent Document 1). However, a Ru-based dye uses an expensive, rare metal element Ru, and thus has problems of high production cost and limited resources. Accordingly, there is a demand for the development of a sensitizing dye which is lower in cost and whose resources are less limited.
It is a well known fact to those skilled in the art that porphyrin is a compound useful as a sensitizing dye because of the ease of molecular design, the high extinction coefficient, and the low cost of synthesis. In the case where porphyrin is used as a sensitizing dye, its molecular structure basically includes a porphyrin ring and an anchor unit introduced therein (e.g., in a porous layer of particles of a metal oxide such as TiO2 formed on the surface of a transparent electrode, a unit that functions to allow the sensitizing dye to be adsorbed to the surface of each metal oxide particles). As such a porphyrin derivative, for example, 5-(4-carboxyphenyl)-10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrinatozinc(II) having a carboxyphenyl group as an anchor unit at a meso position (abbreviated as ZnP), as represented by the following chemical structure formula, is known. However, this compound has poor ability to collect light at a wavelength near 500 nm and also at a wavelength of 600 nm or more (Nonpatent Document 2). As a method for improving the insufficient light collection characteristics of such a porphyrin derivative, for example, a method in which a further substituent is introduced into the porphyrin ring for the purpose of optimizing the molecular structure is mentioned. However, its effectiveness has not yet been completely proven.
